text:
How is Airbus A320 cockpit philosophy designed?
I believe there is a reason behind a particular design especially in aviation.
I wanted to know how A320 cockpit was designed ?.
For example: Why is RCDR, EMER ELEC PANEL is on the left side of the OVHD panel (is it because the Captain has priority/authority to perform these actions ?)
How are the buttons designed?
for example:

As you can see Cargo smoke fwd compartment has only indications and it is not really a button
whereas if you see ventilation panel OVRD is enclosed in square box. What does it exactly indicate ?.
I am interested in knowing the thought process went into design engineer for the reason behind this development.
Any references to external document would be highly appreciated!

Kind of surprised the words Feng Shui weren't used....

I'm not specialized in Airbus, but here is a general answer for a part of your question:
The logic in cargo smoke panel is such, that the black button is an indicator. If smoke is detected by both sensors, the"SMOKE" text will illuminate. Crew will then verify the alarm, and co-pilot will lift the red latch and activate the fire bottle by pressing the button under the latch. The button on the right will then verify the fire bottle discharge by illuminating the "DISCH" text.
This video will explain how the CARGO SMOKE panel works.
The logic here is to separate visually and physically the indications and operating of a critical system. This will mitigate the possibility of mistakes, which is of great importance, since (as DeltaLima pointed out in the comments) the  discharging of a fire bottle is not reversible, you only have one chance / bottle.
The ventilation system is not (as) critical from safety point of view, hence indication and operating can be integrated. Under normal condition, which is "AUTO" the button is not illuminated. Pilot can shut of the system by pushing the button which will then illuminate the "OVRD" text. If the system detects a fault, obviously the "FAULT" text will illuminate.
As for the placement of separate panels, they are generally placed such that they form logical groups and they are as close as possible to the crew member that is usually supposed to operate them.

You may want to emphasize the irreversible nature of the discharge button. Once pressed, the content of the single bottle of fire extinguishing agent is fully released into either the forward or aft cargo bay. Unlike the blower en extractor fans, which can be switched on and off without further consequences,  the fire bottle discharge is a one-shot operatation which cannot be undone. If the wrong bay is selected, or the button is accidentally pressed, there is no way to correct that mistake. Hence a red guard latch to offer protection against accidental activation.

I'm not into cockpit design so here is just a (partly) educated guess for the main layout.
If you do "boot" the plane from Dark-Start up of the plane you have to check, if all white "off" lights are unlit before taxi.
So the design of e. g. the Fuel Pump buttons includes only a white "off" lamp and not an "on" indicator.

On the other side e. g. the APU Master button only has a blue "on" indicator.

The Fuelspumps should be activated before Taxi, the APU is not required to be turned off when taxiing.
So the button design of these simplifies the procedures and makes overseeing the activated functions or functions that still have to be enabled easier.


text:
What frequencies are used to transmit ADS-B messages?
Presently, ADS-B messages are broadcast on

1090 Mhz Mode S Extended Squitter
VHF Data Link Mode-4 (VDL-4)
Universal Access Transceiver (UAT) 978 Mhz

First 2 Frequencies have relevant ICAO standards and third one is predominantly used in America.
Out of the first 2, how to identify in which frequency ADS-B messages are transmitted ?
Also, Presently how many Air Traffic Control around the world use ADS-B messages as means of enforcing surveillance ?.
If installing ADS-B receivers (ADS-B IN) and relevant software are cost-efficient, (Like Flightradar24), Why many ATCs aren't making the switch and if ATC are using ADS-B messages, why do ATC still assign Squawk code ?.

I think you have the answer in your question for 1 and 3. 1090 MHz and 978 MHz. VHF data link operates on aircraft VHF frequencies. You might want to try a cleanup on your question to focus on a single topic at a time.

1090 MHz Mode S Extended Squitter. This is broadcast on 1090 MHz. It is a capability embedded in virtually all current Mode S transponders. The transponders also support Enhanced Surveillance which allows ATC to request most ADS-B data fields (AKA: BDS Registers) directly via Mode S datalink. In some areas ADS-B ground stations are used to provide ATC surveillance in areas where radar coverage is unavailable (mostly mountainous areas.)
VDL Mode 4 has been in development in the northern European area (predominantly Sweden). It operates within the VHF aviation band, initially 118 MHz to 137 MHz. From a 2011 report from ICAO, the operating band has been expanded to 112 MHz to 137 MHZ. Frequencies are assigned by ATC.
UAT operates on 978 MHz.

As ASE is based on a one question-one answer concept, please post your other questions separately.

Are they share the same modulation and physical layer structure? and the only difference is carrier frequency?

@mohammadsdtmnd No, they operate with different protocols and structure. I would suggest a search as there has been a lot of data published and then ask specific questions separately if you still have questions.

source: Stack Exchange

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text:
Are Aerodrome Reference Code used only for Airport planning purposes?
Excerpt from Annex 14-Aerodromes

"1.6.1 An Aerodrome Reference Code - code number and letter - which is selected for aerodrome planning purposes shall be determined in accordance with the characteristics of the aeroplane for which and aerodrome facility is intended."

Is the aerodrome reference code used only for aerodrome planning purposes and once the aerodrome has been built, aerodrome reference code is not used ?.
for A320/A321 - Aerodrome reference code is 3C/4C, are these codes determined only to plan an aerodrome and not used in day-to-day operations ?.

The code determines the capability of the aerodrome in term of aircraft size. Therefore operators must check they land their aircraft on compatible aerodromes. This is done when opening new lines or adding new aircraft to an existing line.

Source.
The information is provided for the aerodrome and the aircraft and publicly available. In the table above you see also other compatibility information:

Approach category
RFF and ARFF (rescue and fire firefighting)

You can read more in:

Boeing Aircraft and Airport Compatibility
CASA Aerodrome and aircraft compatibility

From CASA document:

When introducing new aircraft types or subtypes to an aerodrome the following should occur:
− the airline/aircraft operator submits a request to the aerodrome operator to operate a new type of aircraft to the aerodrome.
− the aerodrome operator should do a compatibility study to determine if the new aircraft type can be accommodated. This will include provision for access to the movement areas, and if necessary, consider the feasibility and economic viability of upgrading the aerodrome infrastructure to meet the requirement of the new aircraft type.
− both the aerodrome operator and aircraft operator should discuss the aerodrome operator's assessment. The assessment should identify:

likely resultant consequences that may evolve during operation of the new aircraft type
whether the aerodrome will be able to accommodate the aircraft size, width and operational characteristics
the conditions under which it will operate safely.

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text:
What is the aerodynamic benefit of trailing edge sweep?
Sweep will allow flight at higher speed because the perpendicular air flow speed (over wing) is less.
But why is there trailing edge sweep?
We can have a swept leading edge and unswept trailing edge to produce elliptical lift distribution. Also, unswept trailing edge will help to improve flap effectiveness, right?

Both leading and trailing edge sweep help to reduce compressibility effects. For simple estimates, quarter chord sweep is used. This is more influenced by the leading edge, but the trailing edge is still involved with 25% of the total value.
If we look at the particular influence of trailing edge sweep, it is mostly found in the effectiveness of flaps and ailerons. Their effectiveness is proportional to the cosine of hinge line sweep. However, flaps at the end of a long wing chord will create a high pitching moment, requiring a big tail. Most delta wings use simpler, less effective flaps with less moment change.
In supersonic aircraft trailing edge sweep helps to reduce sonic boom intensity.
In the end, trailing edge sweep is mostly an outcome of the design process, similar to wing chord. The structural engineer will want to have a deep root airfoil in order to have enough wing height for a stiff spar or the landing gear without increasing relative thickness too much, and the aerodynamicist wants to reduce wetted area, since it will create friction drag and increase wing mass. The stealth engineer will align the trailing edge with other edges or at least put substantial sweep on the trailing edge to reduce the frontal radar echo. In the end, all have to agree to a compromise.

I would say that the trailing edge sweep does not have a very large influence on the compressibility effects, as the higest velocities (and thus the strongest shocks) will not occur at the trailing edge?

@ROIMaison: In supersonic flight you have a shock at the trailing edge all the time. Maybe it is not strong, but then the wing does not create much lift. Remember what was planned to reduce noise emissions of supersonic passenger jets: This focused a lot on trailing edge sweep!

Ah right, I was thinking about transonic flight.

For transonic flight the trailing edge sweep is not a design variable, it mainly follows from the quarter cord sweep (to control the effective Mach number) and the taper-ratio (which follows from a compromise between aerodynamics and structural weight, see this answer this answer for a more detailed explanation).

authors: محمود محمد حلمي

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text:
How is altitude reached by aircraft flying above the stratosphere measured?
Aircraft flying above the stratosphere (above 30km) are rare, but there is still the X-15 and the spaceship two. For those aircraft, maximal altitude is measured to within 500 ft.
As described in this question, barometric altimeters are not accurate nor practical that high. Yet, those aircraft are aircraft and as such I expect them to be fitted with barometric altimeters.
How is altitude measured with such precision (X-15: 354,200 ft in 1963, 15 years before GPS)?

Presumable the X-15 was being tracked by radar, which (if properly arranged) is capable of measuring altitude with considerable accuracy.

Bear in mind that although we call these instruments "altimeters", they are really barometers.  They really measure pressure, not altitude. They display altitude. But think about it. The only time we really care what the actual absolute altitude is is when we're close to the ground, like on an instrument approach or navigating in IMC over terrain. The only thing we care about at 40,000 feet is relative altitude, that is our altitude relative to the other nearby aircraft. That's why we don't set local QNH over FL180.

They used inertial altitude on the X-15 for high altitude measurements. This works just like an IRS (Inertial Reference System), which was used on airliners of that era: you can get the position (latitude and longitude) based on a manually entered (or GPS derived) start position by integrating rotations and accelerations over time. The principle also works in 3D, if you also enter the start altitude. Note however that inertial altitude (true altitude above mean sea level) is not equal to barometric altitude, which can deviate based on local environmental conditions.

On the instrument panel was an attitude indicator whose source was inertially derived. Also provided were inertial velocity, altitude, and rate-of-climb. Barometric instruments included an altimeter and a combined Mach-airspeed indicator; these instruments were usable only below about 75,000 feet and were used mainly in the traffic pattern. An angle of attack and a sideslip indicator were provided; the source of these two instruments was selectable, inertial for high altitude flight and barometric for the end of the mission.

(NASA - Dryden History - X-15 Lessons Learned, emphasis mine)
You can see both instruments in the following image:

(modified from source)

Theodolites operated by ground observers. Radar. Inertial instruments.

From [answer]: "Brevity is acceptable, but fuller explanations are better"

Could you add references and links for further readings?


text:
Why do the Comet's engines point outwards?
As it's visible from the images below,
when looking at the plane from the top, the engines point outwards and they are banana-shaped, why is that?
(unfortunately I couldn't find a real photo straight from the back of the airplane, from that angle it is way more noticeable),
https://upload.wikimedia.org/wikipedia/commons/9/93/De_Havilland_Comet.svg

Welcome to aviation.SE! Does this answer your question?

I don't think that answer does it because the engine are buried well back in the wing so that flow orientation is controlled at the front of a long duct.  I'd say it has something to do with angling the thrust line slightly closer to the center of mass for engine failure purposes, or to direct the exhaust flow away from the fuselage perhaps.

The angling of the engines on later Comet models was to prevent engine-exhaust-impingement-caused damage to the aft-fuselage skin, which had been a problem with the Comet 1:

(f) The possibility of damage by jet efflux

During the operation of B.O.A.C. services, there had been some experience of small damage to the cabin skin, due to the buffeting by the efflux from the jet engines. This damage was partly in front of and partly behind the pressure dome of the cabin. As soon as it was observed, a systematic inspection was made of all Comets, and where any signs of cracking were detected a repair was made according to a scheme specially devised by de Havillands. Internal inspection showed that the buffeting was also causing slight loosening of the joint between the stringers and the skin in this region, and rivets were therefore inserted in order to ensure that this would not give rise to danger.

This point of possible weakness was under continuous observation. The steps taken to deal with it may be considered to be satisfactory, particularly since, where the repair had been carried out, no further trouble occurred.

It is, however, recognised by de Havillands that a situation in which it is known that such cracks are likely to occur is unsatisfactory, and among the improvements they intend to make on future Comets is one which they believe will reduce the cause of this damage, namely, a slight change in the direction of the jet pipes at their exits, with the object of diverting the jets away from the sides of the cabin.

[Report of the Court of Inquiry into the Accidents to Comet G–ALYP on 10th January, 1954 and Comet G-ALYY on 8th April, 1954, Part XIII, section (f), paragraphs 112-114 (on page 25 of the report itself and page 45 of the linked PDF of the report), my emphasis.]

The engines were mounted well inboard to reduce the hazard of asymmetric thrust.

Four turbines are placed so close to centerline of plane that even if two on one side cut out, pilot has little trouble maintaining straight, level flight.  [photo caption, p. 101]
...
The Comet's engines are snugged in so closely to the fuselage that there's no need for the customary trim tab on the rudder. [p. 102]

-- Francis, Decon. "I Saw This Jet Liner Fly 500 m.p.h." Popular Science, 156(5), May 1950, pp. 98–104.
Protecting against that hazard also explains the deflection of the engines' exhausts, which places them closer to a line intersecting the airplane's center of mass.
Another reason for the deflection could be to prevent jet blast from overheating or otherwise damaging the fuselage's skin just aft of the exhaust.  The dH Ghost turbojets were built a decade before the first high-bypass engines, so their hot exhaust had no surrounding cooler air.
A third, faint, reason might be to facilitate egress from the over-wing emergency exit windows, sliding aft and down instead of climbing forwards.  But avoiding asymmetric thrust is the reason that is documented in the literature of the time.  (By the way, that entire Pop Sci article is a hoot.)

It is a very cool answer, but don't you loose a lot of thrust if you vector it a bit sideways instead of all  backwards? And how is it acceptable to lose it just for the reasons you stated?

@Salomanuel: The loss is negligible, as it is proportional to the cosine of the toe-in angle.

Asymmetric thrust does not appear to be pertinent to the question. It explains the lateral position of the engines, but not the orientation. The next-to-last paragraph in this answer seems plausible, but a good answer would include some reference that actually documents the claim. This reads more like speculation.

@PeterDuniho: The orientation of the engines does help with asymmetric thrust, as it angles the individual engines' thrust vectors so they pass closer to the aircraft's center of mass, reducing the yawing moment if one of them cuts out.


text:
In Jeppesen Approach charts, what does D2.8, D3.8 mean?
In Jeppesen Approach chart for VABB VOR RWY 32, what does D2.8 (BBB VOR), the MDA is lower at 650ft. when at D3.8 (BBB VOR), the MDA is higher at 780ft.
How to decide which DME should be taken before commencing the approach ?.
What parameters should be taken to make the decision to choose DME from VOR (2.8 or 3.8) ?
Also, What does 980ft below 2.8D (BBB VOR) mean in approach profile view?.

The title doesn't seem to match with the content of your question. So what is your question?

The points D1.0, D2.8, D3.8 and D9.5 all refer to DME distances in units of nautical miles. In this case there is only one DME for this approach: the Mumbai VOR/DME (BBB).
You would fly this approach as follows:

You cross the VOR/DME at 3800' or above and fly outbound (either on radial 151 or 161, depending on your aircraft approach category) while descending towards 2600'.
At a distance of 9.5 NM from the VOR/DME you start the turn and establish yourself on the inbound course of 321° towards the VOR/DME.
At a distance of 7.5 NM from the VOR/DME you start a 3.29° descent from the platform altitude of 2600' (this is the CDFA: continuous descent final approach).
You must maintain 1330' or above until a distance of 3.8 NM from the VOR (gray area below that segment on the profile view). If you cannot identify1 D3.8, then 1330' is also your MDA (right most box in the minimums section). If you follow a perfect 3.29° descent path, you should also cross D3.8 at exactly 1330' (that is the number directly below the point on the profile view).
You must maintain 780' or above until a distance of 2.8 NM from the VOR (gray area below that segment on the profile view). If you cannot identify1 D2.8, then 780' is also your MDA (center box in the minimums section). If you follow a perfect 3.29° descent path, you should cross D2.8 at exactly 980' (that is the number directly below the point on the profile view).
After D2.8 you must maintain 650' or above until visual (MDA in the left box in the minimums section).
At a distance of 1.0 NM from the VOR/DME you reach the missed approach point (M on the profile view). This is the latest point2 at which a missed approach must be initiated, if not visual.

1 The chart says "DME required", but a suitable RNAV system (e.g. based on GPS) can be used instead. This can lead to a situation where you cannot identify the points D3.8 and D2.8 if the corresponding RNAV points 38VOR and 28VOR (written in square brackets below on the profile view) are not on your flight plan.
2 Note that this happens after reaching the MDA on the CDFA, therefore you would normally execute a go around earlier. Also note that the MDA must be adjusted for height loss when used as a decision altitude (DA) on the CDFA (see note 1 at the bottom of the chart).

Ah, this now makes sense with the (1) note at the bottom about VNAV

According to the link you shared (Boldmethod), if 38VOR and 28VOR waypoints are not in the flight plan "and" we don't have DME equipment, then our minimums would be 1330ft.
But
if we dont have 38VOR and 28VOR in our Flight plan "but" we have DME equipment on board, then our minimums would be 650ft and we have to fly laterally selected and vertically selected (in terms of flying Airbus A320)?
Can you also confirm if Laterally managed and vertically selected approach is possible if 38VOR and 28VOR is unavailable ?.

@KrishnaKanth In an A320 you would typically fly any VOR approach in lateral and vertical managed modes. This would either be FINAL APP or FLS (if installed). If you don't have those RNAV points you can still tune the VOR and manually check the DME distances. Otherwise, you'd have to set the higher MDA of 1330' (plus adjustment) as your minimums. See also https://safetyfirst.airbus.com/safely-flying-non-precision-instrument-approaches/

@Bianfable Can you please explain
If the chart says "DME required" and an aircraft does not have DME Equipment but only has GPS receiver, where in the documentation says this approach can be performed using GPS ?.
2. for example, in an A320, "38VOR" waypoint is only available in F-PLN, our technique would be to set our minimums to 650+height adjustment and then when the aircraft crosses 38VOR waypoint, we can switch to vertically selected, why do you say that we can perform lateral and vertical managed beyond "38VOR" ?.

@KrishnaKanth Whether GPS may be used instead of DME probably depends on the specific country policies, in the US AIM 1-2-3. I'm not familiar with Indian regulations. Airbus says "Managed modes are recommended, but selected mode might be useful in case of system or equipment failures" (link above). I don't know what policies your airline has, but I'd use managed modes down to minima (FLS can also be used below minima) and then continue visually. Feel free to ask a new question on how to fly a NPA with an A320.


text:
Does RNAV and RNP specify what type of longitudinal accuracy the system has?
I understand RNAV 1 to mean it will be within 1 mile of the desired track 95% of the time, but that is lateral accuracy, what about longitudinal accuracy, in front and behind the estimated position along the route. Does the RNAV specification give us any guarantees for that?

The number indicates that the equipment knows where you are at within a circle of that radius 95% of the time.  The CDI deviation limits shown on the PFD HSI also corresponds to the RNAV number.  All certified equipment (c129 or c146) are required to inform the crew when accuracy is not within those RNAV limts.

It does, but it's a different issue than just positional accuracy. The goal of RNAV is to keep the aircraft within the confines of the specified airspace. Longitudinally, you will always be within the airspace if you're meeting the cross-track and vertical containment as you're moving along the defined path.
What really matters longitudinally is when you reach a point along the path. Since airspeed, winds, and aircraft performance affect this it is very dynamic. And it's not just an absolute time, ATC can adjust these targets to keep aircraft properly spaced along the route. This is accomplished using 'Time of Arrival Control' requirements.
RTCA document DO-236 Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation provides the following guidance:

1.3.2 Containment Methodology
...
An aircraft meeting the required Time of Arrival Control requirements
will satisfy the lateral and vertical confines of RNP RNAV/VNAV
airspace while meeting a required time of arrival. Application of this
MASPS is intended to assure that systems will achieve required times
of arrival within 30 seconds 95% of the time.
...

and

1.4.6  Time of Arrival Control (TOAC)
The CNS/ATM operating environment is expected to utilize aircraft
capability to improve traffic management for route crossing and
metering operations. The use of time of arrival control is intended to
provide an increased certainty of an aircraft arriving at a fix at a
required time. TOAC as the time element for RNP navigation will not
change the system navigation performance. TOAC may alter, where
allowable, the lateral and vertical paths in space, and associated
containment/vertical path performance limits to reflect changes in
aircraft path and speed.

Unfortunately in the US many of the regulations are written to make it difficult to separate accuracy/integrity in the along-track direction from the cross-track direction. DO-229D claims this is "since RNAV positioning accuracy is generally independent of the aircraft track.", but I have to scratch my head at that since using a circular window only makes sense in some circumstances. Latency effects and DME/DME accuracy for example can be much greater in one direction than another. As @JanHudec points out though, when considering changes of direction the circular accuracy is more useful and intuitive than, for example, separate measures of NS and EW accuracy.
For example DO-229D, "MASPS: Required Navigation Performance for
Area Navigation" says:

The Horizontal Protection Level Fault Detection (HPLFD) is the radius of a circle in the horizontal plane (the local plane tangent to the WGS-84 ellipsoid), with its center being at the true position, that describes the region assured to contain the indicated horizontal position.

DO-236B and DO-229D have similar language for the Radius of Containment.

In this image protection level would be R1, or R2 if being conservative. This image is not regulatory, since this is just from a class discussing position integrity.
DO-229D does describe in its appendix some scenarios though where the position integrity is not necessarily circular:

For en route through LNAV approach, HPL_SBAS must bound horizontal radial position error with a probability of 1 - 10-7 per hour; i.e., the probability that horizontal radial position error exceeds HPL_SBAS must not exceed 10-7 in any hour, except possibly for brief periods less than the time-to-alert. For LNAV/VNAV, LP, and LPV approaches, the probability that horizontal cross-track error or vertical error or both exceed their respective protection levels must not exceed 2x10-7 per approach. Only one dimension is used for HPL_SBAS in LNAV/VNAV, LP, and LPV approaches, since the along-track tolerance is so much larger than the cross-track. The worst-case dimension is used.

Also, keep in mind that for climbs and descents, large along-track directional uncertainty will mean you're not sure how close to the vertical path you are. AC 90-101A says that for RNP AR approaches like the one at Aspen equipment should account for this coupling and warn the pilot if uncertainty about where you are along the path could lead your altitude to be too far above or below that path.

“Latency effects and DME/DME accuracy can be much greater in one direction”, but that direction is determined by the location of whatever navigation aids you are using, not the direction of flight, so “positioning accuracy is generally independent of the aircraft track” still holds. Also if there is a turn, longitudinal error will make you turn early/late and become lateral error after the turn. So it is the upper bound of error in any direction, i.e. the circular window, that matters.

@JanHudec As you suggest it's more complicated than my simplistic statement, and I'll probably figure out a way to include your feedback. On straight segments it seems silly to fail RNP if you're using two DME stations at almost right angles to the current track and therefore have bad along-track accuracy.
By "latency", I mainly mean latency of sensors on the plane between measurement and display or alert.  When that latency is converted to uncertainty (with or without extrapolation to compensate for the latency), the latency adds a different amount of uncertainty along track vs across track.


text:
How much obstacle clearance is provided within full scale deflection below glide slope on ILS?
Basically the question in the title. The military has regulations that say to obey the localizer minimums if more than have scale deflection below glide slope, but for civilians how much obstacle clearance is provided if they are below glide slope and not quite full scale deflection?

In the USA, FAA Order 8260.3 specifies the Standard for Terminal Instrument Procedures. The defining concept of terminal procedures is the Required Obstacle Clearance (ROC) between the nominal flight path and the Obstacle Clearance Surface (OCS). From Paragraph 2–1–4a:

The obstacle evaluation method for descent on a glidepath is the application of a descending OCS below the glidepath. The vertical distance between the glidepath and the OCS is the ROC; thus ROC = (glidepath height) - (OCS height). The ROC decreases with distance from the PFAF as the OCS and glidepath are converging towards the landing surface (see Figure 2–1–3). The OCS slope and glidepath angle (GPA) values are interdependent: OCS Slope = 102/GPA; or GPA = 102/OCS slope.

Note that the TERPS convention is to define "slope" as "run over rise." Thus a standard glide path angle of 3.00º corresponds with an OCS angle of 1.685º.
This is almost enough to answer your question, which boils down to: What is the height of full-scale-deflection-low above the OCS? In order to answer that, we also have to know at what point the OCS begins.
Chapter 10 of the 8260.3 deals with precision approaches (and LDAs with glideslope). From 10–2–1a, the OCS associated with the Final Approach Segment is divided into the "Y," "X," and "W" Surfaces, mirrored about the Final Approach Course. The "W" and "X" surfaces are considered the primary area and the "Y" surface is considered the secondary area.

Figure 10–2–1

Side view of the runway threshold. Click to enlarge.

The rising W surface begins at a point given by Formula 10–2–2:
$$
d_\mathrm{origin}~(\mathrm{ft}) = \max \left[ 200, 1154 - \frac{TCH}{\tan \left( GPA \right)} \right] \tag{1}
$$
There are many pages of instructions on how to determine the actual slopes of the surfaces according to the known obstacles; the TERPS specialists will adjust the TCH and OCS slope to find an acceptable compromise which keeps the GPA within the limits imposed by Table 2–6–1. But we don't need to concern ourselves with that; we know the resulting numbers already because they are published on the instrument approach plate.
So we can calculate the Required Obstacle Clearance at a point $x$ feet from the threshold (where $x$ is equal to or greater than $d_\mathrm{origin}$):
$$
\begin{align}
H_\mathrm{GP} (x) &= x \cdot \tan (GPA) + TCH \tag{2} \\
H_\mathrm{OCS} (x) &= (x - d_\mathrm{origin}) \cdot \tan \left( \cot^{-1} \left[ \frac{102}{GPA} \right] \right) \\
  &= (x - d_\mathrm{origin}) \cdot \frac{1}{102 / GPA} \\
  &= (x - d_\mathrm{origin}) \cdot \frac{GPA}{102} \tag{3} \\
ROC (x) &= H_\mathrm{GP} - H_\mathrm{OCS} \\
  &= x \cdot \tan (GPA) - (x - d_\mathrm{origin}) \cdot \left( \frac{GPA}{102} \right) + TCH \tag{4}
\end{align}
$$
For example, consider the "typical" ILS approach with a GPA of 3.00º and a TCH of 50', leading to a $d_\mathrm{origin}$ of 200'. At a point 1NM from the threshold,
$$
\begin{align}
ROC (6076) &= 6076 \cdot \tan (3.00º) - 5876 \left( \frac{3.00}{102} \right) + 50 \\
 &= 318 - 173 + 50 \\
 &= 195~\mathrm{ft}
\end{align}
$$
But we aren't looking for nominal obstacle clearance; we want to know the obstacle clearance at full-scale deflection. The FAA's Instrument Flying Handbook implies (but does not say outright) that full-scale deflection is 0.7º off of GPA. Thus:
$$
\begin{align}
H_\mathrm{FSD-} (x) &= x \cdot \tan (GPA-0.70º) + TCH \tag{5} \\
OC_\mathrm{FSD-} (x) &= x \cdot \tan (GPA-0.70º) - (x - d_\mathrm{origin}) \cdot \left( \frac{GPA}{102} \right) + TCH \tag{6}
\end{align}
$$
Again considering the typical ILS approach from before, the function is bounded by
$$
\begin{align}
OC_\mathrm{FSD-} (50200) &= 50200 \cdot \tan (2.30º) - 50000 \cdot \left( \frac{3.00}{102} \right) + 50
   = 596~\mathrm{ft} \\
OC_\mathrm{FSD-} (200) &= 200 \cdot \tan (2.30º) - 0 \cdot \left( \frac{3.00}{102} \right) + 50 = 58.0~\mathrm{ft}
\end{align}
$$

Note that the above numbers only hold true when the aircraft is within the W surface. The X and Y surfaces rise higher than the W surface, therefore obstacles may be closer to the glide slope height and the aircraft will be closer to those obstacles at full-scale-deflection-low.

See also Paragraph 2–6–6, setting the standard for the Vertical Guidance Surface, which "must be evaluated for all PA and APV approach procedures (except helicopter APV PinS procedures)." This Vertical Guidance Surface extends from the Landing Threshold Point (LTP) to the Decision Altitude (DA) point, i.e. it is inside of the expected "pilot flies visually" point.

+1 very nice.  Note that I didn’t check it.  I just trust you.  ;-)


text:
Why is the air smoother when you get above the cloud base?
Why is it bumpier below where the clouds form and once you get above the bases the air tends to be smoothers?

Welcome to Av.SE - nice first question!

Because thermals only reach up into the clouds.

The question contains a misconception.  Actually the air can be very bumpy inside clouds.      It is more accurate to observe that the air tends to be smooth above the cloud tops than the cloud bases.  Of course if you are above cloud base and are not inside a cloud, then you are not in an updraft, but you might conceivably be in a downdraft.  Anyway to a first approximation the cloud tops (not the cloud bases) mark the top of the "mixed layer" where it is turbulent.

In some sense the question is backwards.  But, that really leads to the answer.  Clouds form when there is air movement.  For example of how clouds can form: when there is local heating, such as due to the sun heating a large black parking lot, the air over that will rise.  Flying through that air is not as smooth as in other places.  As that air rises up it expands and cools leading to an increase in the relative humidity.  If it does that enough it reaches 100% RH and a cloud forms.  But, that also leads to a decrease in the energy available to move that air up, so it doesn't keep rising up.  So, the air above clouds tend to not have as much energy and movement as below.
So, clouds form where there is air movement, and they "mark" that movement.

A suggestion to improve this answer-- actually, the latent heat released when the water vapor condenses into a cloud releases more energy into the column of rising air,  decreasing its rate of cooling and thus further stoking the thermal updraft.  (Google "wet adiabatic lapse rate" and "dry adiabatic lapse rate".)  This can be an important factor in the formation of thunderstorms or towering cumulus clouds.  Still it remains the case that the air is smooth above the cloud tops because the cloud tops are also the tops of the thermal updrafts.

Re "But, that also leads to a decrease in the energy available to move that air up, so it doesn't keep rising up. " -- see prev. comment-- actually, to understand why the air eventually stops rising, you have to look at the density (temperature) vs altitude profile of the surrounding atmosphere.

The fundamental problem with this answer is that it suggests that cloud formation tends to stifle updrafts, which is not true at all (except of course for effects relating to shading of the ground.)


text:
Why don't companies or innovative engineers find affordable and reliable ways to manufacture tried and true designs?
Cessna no longer makes the 150 or 152. It is a shame because the aircraft is reliable and a great plane. With such a tried and true design, what is stopping a different company from using the same blueprints, and making brand new Cessna 150s again. I am asking this because, as most pilots do, I often lament the prohibitive cost of general aviation. 60 years after the production of the 150, Cessna has decided to pursue creating more expensive products rather than refine the manufacturing processes to make their classic models more affordable. It seems like one way we can make aviation affordable is for pioneering individuals to develop efficient and cheap manufacturing methods to start producing again the models that have already been flight tested and certified. What am I missing here. Why has nobody tried to do this?

How would you propose one would acquire the necessary drawings, manufacturing tools, etc?

Someone would have to talk Cessna into selling the type certificate and tooling for the 150, and they probably aren't interested.  Part of the problem is there is still too large a pool of old ones that can be restored more cheaply than building new ones.  Anyway, it's a myth that aviation used to be cheap.  A  new 172 for 22 grand was the cost of a pretty nice suburban house in 1969, or 4 to 5 new cars.  I was paying 30+ bucks an hour to rent one in 1976 when 5$ an hour was decent salary.  That was a LOTTA money at the time.  I didn't think I was living in a golden age of cheap flying then.

There would be copyright issues, unless they got permission from Cessna.  (Though note that there are clones of Piper Cubs.)  Besides, why would you want a copy, when for about the same money you can build something better, using modern tech?

Note that they are still making the more popular 172.

I think you are underestimating a few things,

It's already common practice to license a design to other manufacturers. For example, Reims Aviation used to produce a large number of Cessna derivatives.
Initial sale price is only a part of the total cost of ownership. Fuel use, maintenance costs and insurance make up a large fraction of the total costs. Aircraft depreciate relatively slowly (compared to e.g. a car).
Old planes are designed for old methods of manufacturing. It's not an easy task to reduce manufacturing costs without modifying the design, which would require new certification.
While you say it's a 'great plane', you really should say it's a 'great plane for its time'. New planes are typically better in terms of gas mileage, cruise speed, avionics, passenger comfort, safety, noise, looks, time between overhauls...

Regarding the Cessna 150 specifically, a 150 cruises at only 95 knots. I wouldn't spend money on a newly-built expensive version of something that slow when I could buy something newer and faster like a Diamond DA-20 (used or new, depending upon your budget), which cruises 20 knots faster while burning the same 6 gallons of fuel per hour.
I'm probably not alone.


text:
Why did Boeing not create a new 737 instead of just upgrading the old model?
This documentary claims that

James McNerney - Boeing's first CEO without a background in aviation, made the decision to upgrade the 737 series to 737 MAX instead of developing a new model.

Is this true? If so, why did Boeing make the decision not to make a new model instead of upgrading a 50 year old one?

@Cloud What do you expect from an answer: another source that claims the same thing the documentary claims (like this article)? Or actually someone who was in the room with him when he said it?

@Bianfable first hand account would be good or at least, a reason he chose that option

It really depends on what you call a "new model".

There is literature discussing why Boeing did what they did... the present version of the question looks good to me. Clear, answerable with facts, on topic.

As the other answers mention, Boeing management was complacent and were caught with their pants down by the Airbus 320 Neo. Upon its birth, Airbus industries took on a whole new, and radical, if I may say so, approach to airplane design. From the very beginning they produced a family of aircraft, where Boeing with its payload of history made more "unique" planes, in lack of a better word. Boeing planes are more like cousins to each other, not siblings.
This ideology of developing a plane family enabled Airbus to develop their concept very efficiently, and their progress came as somewhat of a surprise to Boeing. They were in trouble, as their R/D process is not as streamlined as its European counterparts. One of the major traps Boeing had already fallen into on its path quite some time ago, was that 737 was such a successful plane. They thought it would, and wanted it to, last forever. They thought the MAX upgrade was just another step to be taken to stay competitive, and they could fast track it if necessary. Needless to say, hindsight being 20/20, they were terribly wrong.
The reason 737 MAX failed big time was the fact that quite unfortunately, while Airbus 320 upgrades were building up unbearable pressure on them, a company culture formerly foreign to such a safety oriented industry as aircraft manufacturing was setting in at Boeing. Cutting corners, using sub-par outsourcing, hiding problems, misleading officials.... This is well documented on multiple sources. If one follows aviation news at all, it would have been impossible to miss.
Had Boeing stuck to high quality of design work, the end result would have been different. I mean, the 737 is an engineering marvel, there's no question about it. They just seriously effed it up on its last upgrade. There's a saying in the business world: Nothing can save a thriving business. Boeing was doing too well to see it's own faults.
So the factors leading to the catastrophic MAX project descicion  were an unfortunate mix of falling off the R/D wagon a decade (or two) ago, underestimating the competition, and poor understanding of the industry by company management.

Keep in mind that Boeing has 4,283 orders for 737 MAX, which hardly qualifies as having  failed big time. I do agree that they have a huge mess on their hands that has cost the company $18 billion and counting with only a vague idea when the plane will start flying again.

One must also keep in mind that this 18 billion is the sum as it stands now. That's about 4.2 million per plane (if the 4200 ever gets delivered in full). Needless to say, that is a huge blow on the profitability of the MAX program. We have not seen the end of this, for example the consumer reaction when MAX returns to service is still a big mystery.

Other sources confirm the statement from your documentary that James "Jim" McNerney made the decision to build the 737 MAX rather than a new model:

When Airbus was about to land American Airlines with a huge order for the A320 family, both the ceo and neo, Boeing’s hand was forced. Within 48 hours, Jim McNerney, Albaugh’s boss, made the decision to go forward with what would become the MAX.

(Leeham News, emphasis mine)
The 737 MAX program was eventually launched as a response to the Airbus A320neo, the new engine option. If Boeing had started development of a new model immediately when Airbus announced the A320neo in 2010, they might have been able to come up with a competitive product in a similar time frame. But Jim Albaugh, who was head of the 737 program at the time, underestimated the A320neo and said:

I think Airbus will find re-engining the A320 more challenging than they think it will be. [...] When they get done, they will have an airplane that might be as good as the Next Generation 737. We think we can continue to make incremental improvements to the 737 to make sure that it is a more capable airplane than even the re-engined A320.

(Leeham News)
Boeing wasted valuable time here while Airbus was working on the A320neo. Boeing also struggled with the 787 at the same time:

Boeing was indecisive about which direction to choose.
The ambiguity was driven in no small part during this era by the continued 787 debacle, which at the time of this interview, had not entered service.

(Leeham News)
The final decision to re-engine the 737 instead of developing a new aircraft, came when American Airlines decided to purchase a large number of Airbus aircraft. Boeing realized they had to act quickly at this point:

Boeing faced an unthinkable defection in the spring of 2011. American Airlines, an exclusive Boeing customer for more than a
decade, was ready to place an order for hundreds of new,
fuel-efficient jets from the world’s other major aircraft
manufacturer, Airbus.
The chief executive of American called Boeing’s leader, W. James
McNerney Jr., to say a deal was close. If Boeing wanted the business,
it would need to move aggressively, the airline executive, Gerard
Arpey, told Mr. McNerney.
To win over American, Boeing ditched the idea of developing a new
passenger plane, which would take a decade. Instead, it decided to
update its workhorse 737, promising the plane would be done in six
years.

(New York Times, emphasis mine)

I would suggest the same reasons Boeing decided to pursue an update to the 737 over developing a new plane is the same reason Airbus only updated the A320 (a 33 year old plane) instead of building an entirely new platform, time and development costs. Airbus sunk around \$25 billion into the A380 and lost money on the project, since it was cancelled before it broke even. Airbus did not have the capital to pursue both a an A320 replacement and the development of the A350.
There is one other factor that one should consider, any replacement for the 737 is going to be an entirely new plane with new training, new maintenance, new parts. What happens if legacy carriers like Southwest or Ryan who fly only 737 decide to go with Airbus instead? On the surface, an upgrade to the existing 737 made a lot of sense.
Boeing has talked for years about a project to replace its entire civil aircraft portfolio with advanced technology aircraft, called the Yellowstone Project.
To say that several things make such a project daunting is an understatement. There are two factors for launching an entirely new aircraft that come to mind, the cost to build and the time frame to go from announcement to first commercial service.
The Boeing 787 started out as a part of Yellowstone - Project Y2. It was announced in 2003 and went into service in 2011. The total cost for the program with delays sits at \$32 billion.
For a full replacement of the 737, which the development is now being referred to as the Boeing New Midsized Plane, I would assume we would see similar costs and timeline for development. I have heard Boeing plans to release some sort of replacement for the 737 it hopes to have flying before 2030. I would assume such a development would run around \$20-\$30 billion or more.

I disagree on only 1 point: I'd expect a new 737 replacement to cost significantly more to develop than the 787 did. On the plus side, Boeing's already got all the carbon-composite experience and manufacturing know how. On the minus side, costs have only gone up since 2003.

@FreeMan, I would agree the costs are going to be enormous. 787 development rose dramatically with the problems from the batteries, fasteners and suppliers unable to supply what was ordered. There was 6 787s at Paine Field nobody wants because of problems in early development. One hopes Boeing can not repeat those issues.

The Boeing 737 line of aircraft have been one of the most popular for a long, long time. This means that there are a LOT of airlines owning 737s and a LOT of pilots type rated on the 737s, and a HUGE network of supportive industries in the supply chain.
If they came up with a design that is "too" different from the preceding models, then, according to the rules, pilots would have to go through a new type rating process - which is not good news for people who spend the money. Also, it would be a big task for the intermediate industries to prepare themselves to supply the materials required to manufacture the new 737 model. This would make transition from NG to MAX quite cumbersome because a huge network of industries would have to be upgraded.
In fact, if you take a look at the developments since the 737-300 or 737-400 generation, the evolution curve hasn't been too steep, i.e., from a pilot's place, there is not an overwhelming amount of difference between older and newer generation 737s.
I don't know all the reasons why they chose to build the MAX around the older foundation but making the transition to MAX seamless is definitely one of them.
This is also the reason why the new LEAP 1-B engines on the MAX models had to be pushed a bit forward and lifted up (since the 737's height is too low for the LEAP 1-B to be placed at the old spot), and why MCAS had to be introduced on the MAX lineup (to compensate for the changes in the pilot's experience brought about by the new position and increased power of the engines, and due to the aerodynamic effects of the larger surface area of the engine nacelles[1]).
The issue which, over time, became more and more problematic is that the 737 is from the late 1960s, which wouldn't have affected the MAX lineup (or affected less) if a big change had already (at least) once been done some time between 1967 and 2017.
References:
[1] - MCAS on 737 - Wikipedia

There is actually LESS pitching up with power on the Max. Moving the engines up, closer to the C of G, cancelled the increase in power. The pitching up moment is solely aerodynamic  due to the larger nacelles placed farther forward of the C of G.

Boeing had only just introduced the 737 NG and sat back to relax when Airbus announced the A320neo. It trumped the NG and looked like eating seriously into Boeing's planned market. Boeing felt they had no choice but to fight back.
They had a project for a replacement going, but bringing it forward faced an impossible problem; it could not be done in time to hold back the challenge from Airbus, it would have meant a generation of lost orders.
So an emergency upgrade to the NG was the only solution. But even then the airlines wanted to know what it would give them that the neo did not. Part of that package was the next-generation CFM LEAP engine, but Boeing feared the technical uplift would not be enough.
So they hit on the idea of selling a "you already know how to fly it" airplane, which would save the airlines crew conversion costs and mean that Boeing could short-cut the usual into-service transition period. This proved popular with the airlines, and some even wrote penalty clauses into their contracts if Boeing did not make good their promises.
Boeing did not, but that is another story.


text:
Are there any high altitude flying options for a private pilot?
If you are a private pilot that wants to fly really high, what are your options?
For gliders it's not uncommon to reach FL160. Beechcraft Baron can reach FL190. Assume you want to reach FL200 or FL300 or FL400. What are your options each time?
Anything possible for a civilian with a reasonable budget?

Hot air balloons have almost reached FL700!

I'm not sure what you're asking here. A US private pilot can fly business jets if they have the required training and type ratings. So really the only obstacle is money (in the US, at least); "FL400" and "reasonable budget" don't usually go together :)

Is there a particular reason you want to fly high?

A Cessna 421 can reach 30,000’.

There are no regulatory limits on the altitude that a private pilot can fly. Class A airspace requires an Instrument Rating, but that’s just an addition to your PPL, not a new license.
There are many turbocharged piston airplanes that can get into the low or mid-20s, for not much more than a normally aspirated piston. Turboprops cost quite a bit more, especially when it comes to fuel, but they’ll get you to the high 20s or low 30s.
Above that, you’re looking at jets, and that means a type rating—and a lot more money.  Contrary to popular belief, you can get a type rating as a PPL. However, the checkride for any type rating is to ATP standards, so you might as well get at least your CPL along the way, and your ATPL as well if you have the hours to qualify for one. Regardless, that’ll get you to the high 40s or low 50s, depending on the jet you buy.
Beyond that, you’ll have to wait for the next generation of supersonic planes that always seem to be 5 years away. While you’re waiting, be sure to win the lottery so you can afford one—and then several more lotteries to pay the fuel bills.

"Contrary to popular belief, you can get a type rating as a PPL. However, the checkride for any type rating is to ATP standards, so you might as well get at least your CPL along the way, and your ATPL as well if you have the hours to qualify for one."
This isn't true, unless you equate "jet" with airliner. You can get Single Engine Turbine added to your PPL with relatively little tuition (Around 10-15hrs training, plus some ground school).
In the UK it's possible to fly some old jet war birds with just the above

@Dan In the US, at least, any jet requires a type rating. And if you can pass a type rating checkride, you can probably pass a CPL checkride too.

"There are no regulatory limits on the altitude that a private pilot can fly." If you are using a pressurized aircraft, you will need a high altitude rating for levels above 25,000 feet.

@RonBeyer Endorsement, not rating. You can still do it with a PPL.

Yes you're right, but it is a regulatory limit of using pressurized aircraft.

The most cost-effective option for a private pilot to exceed 20K MSL would be to fly a sailplane (glider) at a location known for mountain wave conditions.

Here in the UK you can relatively easily get your hands on (to fly, if not to buy!) some old 60's and 70's war birds. Something like the Jet Provost will get you to a good 35,000ft, and is doable (as in, training and buy-in) for about £15k.
Obviously you need to be a competent PPL prior to that and be lucky enough to get a share going for sale, but £15k to fly a "fast" jet seems like a bargain and I'm sure it's doable elsewhere too


text:
Does any engine combine an air-breathing rocket and a non-air-breathing rocket?
Does any rocket engine combine air-breathing and non-air-breathing modes?
The advantage would be it would have to carry a smaller tank and less oxygen, leaving more room for payload on a weight basis.

Yes.

@Sanchises the SABRE is still in development.

@securitydude5 by definition, rocket engine is not air breathing. You should speak of jet engine.

@ManuH SABRE is described as an air-breathing rocket. That is exactly how its engine functions in airbreathing mode. It has no fans or through-duct, what it has is a rocket chamber fed by fuel and oxidant.

I fail to see why this question is closed. Yes it is an overlap between space.SE and aviation.SE. Aircrafts with rocket engines exist (e.g. the Me163), thus rocket engines are on topic here. Why can't our community accept overlaps?

There are cases when air breaching and non air breaching stages are used (X-15, SpaceShipOne and Two, Lockheed D-21 is air breaching, then pure rocket, then air breaching drone again). But not in the same engine.

@Manu H: I suspect it's closed because (as written) it asks a nonsensical question.  There is no such thing as an air-breathing rocket, BY DEFINITION.  Air-breathing reaction engines are jet engines.  It could be re-written to ask several different questions, e.g. "Is there an engine that switches from jet to rocket?", or "Are there space launch systems that use both jets and rockets?"

There's a very recent claim for an engine called Fenris, which claims to be a no-moving-parts "rocket" that uses ambient air for its oxidizer.
On the face, that sounds like a ramjet, but the claims made by the inventors appear (to me) to be investor bait, rather than anything that could actually be true.  Video on the entry page at the inventor's web site shows a burn more like that of a poorly designed torch, than a rocket or jet.
As noted in comments, there is also the SABRE engine concept -- this is not yet a working engine, as far as I know, but will/would be a hydrogen-fueled turbo-ramjet which uses the liquid hydrogen to liquefy intake air while in jet mod, for use as oxidizer when it switches to rocket mode.  The intent is for use as a spaceplane-to-orbit.

A no moving parts rocket that uses ambient air, it sounds like a ramjet.

@ManuH Go look up the Fenris "rocket" -- their claims aren't  calling it a ramjet, and they seem to think they can convince investors it'll work.  Physically, the engine they have looks much like a rocket thrust chamber and nozzle.

SABRE uses liquid helium to cool the air to near (just above) its liquefaction point. This is then turbo-compressed and fed as oxidant to a conventional hydrogen-fuelled rocket chamber. The intent is to first find sub-orbital applications, with orbital flight being a longer-term aspiration. Currently the individual engine subsystems have all been bench-tested, notably the pre-cooler, and development of a flight demonstrator is starting.

@GuyInchbald Been a while since I looked at the SABRE.  Now they want to carry a second cryogenic liquid?  One they don't even burn off, so have to carry all the way?  Seems like trying to save a concept that didn't work as expected...

@ZeissIkon The helium intermediary has been in the design for a long time now. It has a complex thermodynamic function in utilising the waste heat to help drive the rest of the system. More like making practical a concept that was not quite figured out in the early days. There are other significant changes too, but design details seem scarcer than they used to be.

Had a quick look at Fenris.  It isn't a ramjet.  It gets its air by sucking it in rather than having it forced in by motion.  The idea seems to be that it creates a (partial) vacuum inside the combustion chamber that sucks air in through the the inlet.  I have no idea how that would work.

@JRE poorly, in my opinion.  It's the same idea behind the Gluhareff jet, which is just a high-frequency pulsejet with a fluidic valve.  Requires very high fuel pressure to run a deLaval nozzle to get supersonic fuel flow and entrain combustion air.  I don't see much that looks like thrusting exhaust in the little clip of video I've seen -- not even as much as a Gluhareff produces.

There is a hybrid concept that uses solid fuel in the rocket motor but liquid oxygen as the oxidizer; in this case you can vary the thrust of the motor by varying the oxygen flow rate. in this case you have to carry the oxygen with you however...

Hybrids like that have been around a good while -- they were available for High Power model rocketry twenty years ago (using nitrous oxide as self-pressurizing oxidizer).  But they're still rockets, not jets.


text:
Which kind of stability do aerobatic aircraft have?
I'm designing an aerobatic aircraft similar to EXTRA 330SC.
I would like to know about stability: lateral and longitudinal, static and dynamic.
I read that aerobatic aircraft have neutral static stability.
Some books relate Cm to AoA.  Others relate Cm to CL.
Please detail more about this or provide some bibliography.

When I asked Patty Wagstaff how she was able to make her plane do some of the things it did, as I couldn't even imagine how to move my control yoke to make my tail flip over my nose for example, she said the plane was finely balanced to allow it to do that.

I hope you are designing a model of an aerobatic airplane. If you are building a real one and don't know the basics of its dynamics... I wouldn't one to fly it.

This is a pretty broad subject so unfortunately I have to vote to close it (generally more focused questions work well here FYI) I would recommend the FAA’s Pilots Handbook of Aeronautical Knowledge which has a good primer on the concepts of stability in it.

If you are serious about designing an airplane that will one day fly and certify, you should, to the very least, read up Raymer, Aircraft Design: A Conceptual Approach. It has a chapter on stability & control, and would be a good introduction to the lingos.
More in depth coverage can be found in Etkins, Dynamics of Flight. It's pretty much the bible for S&C engineers.
Once you feel like you have the theory down, design guidelines for conceptual design (even preliminary design) can be found in Roskam, Airplane Design.
Since design goes hand-in-hand with certification requirements, a knowledge of 14CFR Part 23 and AC 23-8C wouldn't hurt.

The farther forward the center of mass is from the neutral point, the more stable the airplane is in pitch.  When the two coincide, the airplane becomes pitch-unstable.  When center of mass is behind, human reflexes are insufficient to maintain controlled flight.
As far as designing goes, center of mass and AoA are different beasts.  Center of mass stays roughly the same, while AoA (especially in aerobatics) changes greatly and quickly.
Others here have explained further.

Did you mean center of lift, or neutral point?

Doh.  NP.  Edited.  Thanks.

Cm is the moment (coefficient). It cannot be 'behind'. If it 'stays the same', the aircraft stability is neutral by definition. Its derivative by AoA, $Cm_{\alpha}$, stays roughly the same (in the normal operational range of AoA), and if it's negative, the airplane is stable. For aerobatics though, the post-stall behaviour is also very important.

Or wait. Did you mean centre of mass? Then write it unambiguously, CM or better CG (even though it's less correct formally), or in full.

Or use math notation: https://math.meta.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-reference As an added bonus, LaTeX uses a very similar system so you gain transferable knowledge in the process.

@CamilleGoudeseune If something in your answer is wrong, don't cross it out -- just remove it. Anyone who wants to know what the answer used to say can check the edit history.


text:
What biplane is this a model of?

What biplane is this a model of?

As noted in @MichaelTracy's post below, this is the night-fighter or "Comic" variant of the Camel. In the picture in Michael's post you can see the two Lewis guns attached to the upper wing spars and the rectangular cut-out ala the Snipe - but both the model picture and the picture in Tracy's post show a straight upper wing and 5 degrees dihedral in the lower wing. So the model is a night-fighter-variant Camel which has had its upper-wing Lewis guns removed.

The plane is probably modelled after both a Sopwith Snipe and a Sopwith Camel. Look at the cutout of the wing above the pilot's seat. It's rectangular as on the Snipe, not a semicircle as on the Camel. The engine looks like a Camel's however.
So, this model is possibly a hybrid of two famous World War I British fighter planes.
It could also be a specially modified Camel according to @Michael Tracy (except the painting scheme does not match the one on the picture he provides).

Have a look at the 6th picture in this Camel link, you’ll see the exact same rectangular cut-out.

The plane is a Sopwith Camel. It was a British fighter in service from 1917 to 1920. There were 5490 built, of which only 8 survive, but many replicas are on static display and a few are even airworthy.

Source

Build an Airfix model of one many decades ago :-).

I thought the Sopwith Camel looked like this

As for the specific plane it appears to be a rough approximation of a Sopwith Camel from No. 209 Squadron RAF–most likely Roy Brown's B7270.

Source: Valder137 CC BY 2.0, via Wikimedia Commons
Capt. Brown was officially credited by the RAF for shooting down Manfred von Richthofen, the "Red Baron", but modern historians suggest it was likely Australian anti-aircraft fire from the ground.

authors: Bob Jarvis - Слава Україні


text:
Database for NACA airfoil polars
Is there anywhere where I can find a database of experimental or CFD-obtained NACA series four or series 6 airfoil data?
I'd like to have access to a relatively large set of high-fidelity drag polars/$C_L \times \alpha$ curves in order to train a neural network for a personal project, and I don't have the time to digitalize the plots in NACA's original contractor reports.
Any help is appreciated. =)

Resource location questions are explicitly off-topic. Please check the [help] for more details.

The technical report NACA-TR-824 has been digitized.  The digitized version includes Cl and Cd for 118 airfoils at 3 Reynolds numbers.  To get it, create an account on x-plane.org.  (It's been online sporadically elsewhere over the decades, e.g. here.  People associated with the digitizing include Gregory Peter, Gregory Siemens, and James Sonnenmeier.)

Convert plots to tables of numbers with Data Thief or WebPlotDigitizer.

At lower Reynolds numbers, http://airfoiltools.com/polar/index lists XFOIL-calculated polars for many of those airfoils, as text as well as plots, e.g. http://airfoiltools.com/polar/details?polar=xf-n0009sm-il-500000:

alpha    CL        CD       CDp       CM     Top_Xtr  Bot_Xtr

-13.500  -0.9606   0.06449   0.06155  -0.0264   1.0000   0.0073
-13.250  -0.9745   0.05737   0.05426  -0.0319   1.0000   0.0074
-13.000  -1.0011   0.04929   0.04597  -0.0377   1.0000   0.0072
-12.750  -1.0177   0.04406   0.04052  -0.0404   1.0000   0.0073
...

You may need some clever webscraping script to collect them all in a NN-friendly format.  Or you could run XFOIL yourself:  they describe exactly how they generated these tables.

Those polars are at very low Reynolds numbers which are often unsuitable for the particular airfoil. Only model airplane airfoils are well represented on airfoiltools. But maybe the classic by Abbot and Doenhoff is a better choice?

Aha!  I didn't know that anyone had digitized NACA-TR-824.  Yes, better.  Propose that as an answer.

I have enough reputation already. Also, it is better to keep all relevant stuff in one answer. So please feel free to add the link to your answer.

What, no upvote so far? This is an excellent answer!

Fear not, google will direct people here in due course!


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Are there advantages to slotted wingtips?
Are there advantages to the way some birds' wingtips end in separate feathers instead of a more solid shape?
As an example, separate-feathered wingtips:

more solid wingtip:

Absolutely there is an aerodynamic advantage and it's not really about "  feeling" the air.  No time to compose a real answer right now.  Not to say that that solution is "better" than albatross but it has some specific points in its favor. It has been tried on some airplanes-- look for related questions on ASE.    More on some other day.

Notice that, besides swallows, seafowl have solid wingtips. The feathered wingtips are found on birds that must fly through branches of trees. They stretch the wing with flexible material that doesn't get damaged when the bird misjudges the available space - similar to a cat's whiskers. The ocean surface and cliffs have no trees, so swallows and seafowl can afford to have the more efficient solid tips.

Continuing on Peter's comment, one can notice that a) seabirds have pointed wingtips not slotted wingtips, and 2) seabirds never have dihedral, and often have anhedral on the outboard portion of the wings, which dominates  the balance of roll torque.  The former point (a) is well suited to high flight speeds and the latter point (b) is well suited to high winds.  Heavy wing loading/ high flight speeds also are good adaptations for a windy environment.

I'll incorporate all this into a real answer in the future!  Anyway, generally speaking, the birds with slotted wingtips are not the ones that fly at high airspeed or more accurately a high "scale airspeed".

But I would also add that while accipiters fly through the branches of trees, Red-tailed hawks, Turkey vultures, and California condors do not.  Neither does the vulture in the top pboto in the question.  So that's not the key variable re slotted tips.  More later.

@q.f., looking forward to these four off the cuff comments and more, in your eventual answer!

Slotted wingtips provide torsional flexibility.

A bird’s wingtip feathers must twist in one direction during the upstroke of the wings and in the other direction during the downstroke to keep the local wind striking the wing at an appropriate angle to generate lift and thrust... The turning could be done at the base, with a completely inflexible feather; the aerodynamics are improved and material saved if the local flow forces twist the feather by just the right amount.

-- S. Vogel, Comparative biomechanics: life's physical world, p. 382, 2003.
They also reduce drag.

The minimum drag of a Harris' hawk gliding freely in a wind tunnel was measured before and after removing the slots by clipping the tip feathers. ... the feathers that form the slotted tips reduce induced drag by acting as winglets that make the wings non-planar and spread vorticity both horizontally and vertically.

-- V. Tucker, Drag reduction by wing tip slots in a gliding Harris' hawk, Parabuteo unicinctus, J Exp Biol 198:775-81, 1995.
They are effective while flapping, not just while gliding.

We used particle image velocimetry to measure the airflow around the slotted wing tip of a jackdaw (Corvus monedula) as well as in its wake during unrestrained flight in a wind tunnel. The separated primary feathers produce individual wakes, confirming a multi-slotted function, in both gliding and flapping flight.

-- KleinHeerenbrink et al, Multi-cored vortices support function of slotted wing tips of birds in gliding and flapping flight, J R Soc Interface 14(130):20170099, 2017.

No wonder induced drag rises after the feathers have been clipped. That is no proof for the efficiency of slotted wingtips. But that stuff gets published nonetheless.

Tucker's "mean induced drag factor" might correct for the shorter wingspan after clipping (the intro discusses clipping vs. gap-filling).  This paper just extends to a living bird his previous "study of a feathered tip on a model wing in a wind tunnel" (1990, J Exp Biol 149, 449–468).  Maybe there's an even better study in the 30 years since then?

They function like an array of winglets.  Each feather is aligned to optimize its angle of attack in the local flow, which is circulating from the bottom to the top around the tip, to extract energy from the circulating flow, weakening the circulation (by redirecting it the other way - a wing deflects air to make lift) and providing a beneficial lift/thrust component ("thrust" to the extent that the airfoil is angled nose down to achieve an optimal AOA, and lift being 90 deg to AOA).
The result is a wing with very low aspect ratio that regains a bit of the efficiency lost with a low aspect ratio wing, where the benefits of low aspect ratio are important for the bird (maneuverability and ability to fight).  I'm sure that these benefits are magnified at the very low Reynolds Numbers that bird wings operate at (to the bird, the air is way more viscous than to the airliner, and to the bee, the air is like motor oil).

Another factor here is that it's much more efficient (lighter, better control, &c) to implement this array of winglets with muscles and nerves, rather than a bunch of electric or hydraulic servos.

Yep.  I'm not religious at all, but the way everything in nature is so precisely "engineered" just leaves me in wonder at the mystery of it all, and very unsatisfied with Darwinist theory.  There is just.. more... somehow. Dunno what.

Maybe a little mathematics will help @John K : using Stetson-Harrison method, we estimate it took ten million years for the different wingtips the contemporary birds to evolve (perhaps under the supervision of the allmighty, who knows), and again, with S-H, we estimate the male and female of the species to procreate at the age of three. So, to this date, there has been a total of 3 333 333 generations of birds since the common ancestor to evolve. It doesn't take that much mutation per generation to produce such a wide variety of our featherry friends.

I'm all in with genetic mutation and evolution and all that, to a point.  Still doesn't explain everything.  For example, DNA is a code. A set of instructions.  A language, really.  Is that random?  There are still questions that science can't answer for sure.  If there was a big bang, what was before that?  And before that?  I just revel in the wonderful mystery of it all.

@JohnK: there is an abundance of stuff in the natural world that is a long way from "precisely engineered", it's hardly "everything".

@whatsisname well if you want to be literalist, yeah... Anyway, living things.  They are "systems".  A cell.  It's an intricate chemical "system". The very first cell of the very first organism on earth.  Where did that come from? Did someone or something design it?  I don't think there's some dude in the sky pulling levers.  But, as to what it's all about, nobody has a clue. It's a mystery.

Birds with wingtips like this tend to be thermal soarers, so they probably help the bird detect air currents. When circling in a thermal, you need to know where the air is rising most strongly so you can avoid flying out of the lift. Glider pilots feel one wing rising or falling, but a bird with delicate feathers could feel the differences between the actual airflow at each tip and respond faster.
The swift in your second photo is not a soarer. It requires agility to catch insects mid-air.
Some birds (like an Albatross) are extremely efficient gliders with a solid wing tip. They glide on much faster moving air that must be easier to feel, so they're shaped for efficiency rather than sensing.

See comment under question--

I've shared a thermal with a soaring bird.  Watching their wingtip feathers respond to very small variations in air currents was one of the neatest things I've ever seen.  It seems as if each feather is adjusted individually to the air hitting just that feather.

@FreeMan - thanks, I've fixed it.

@Wayne each feather does adjust to the local airflow, but as a side effect of aeroelasticity.  There isn't one muscle per feather!  (Actually this robust passive design is more impressive than one with dozens of sensors and actuators.)


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is DME groundspeed slightly inaccurate due to slant range?
When DME is calculating aircraft ground speed, is it doing it based off of the slant range? Wouldn't this mean that if the aircraft is at sea level and the DME is at sea level that the GS would be accurate. But if the aircraft is at altitude, then the slant range is changing differently, so won't the GS be different or inaccurate?

Comments are not for extended discussion; this conversation has been moved to chat.

Your slant range distance to the DME station is given by $s=\sqrt{s_g^2+a^2}$, where $s_g$ is the ground distance and $a$ is your altitude above the DME. Assuming you are in level flight, your computed speed is then $$\frac{ds}{dt}=\frac{s_gv_{gs}}{\sqrt{s_g^2+a^2}}$$
where $v_{gs}$ is the true groundspeed. Assuming that $d_g \gg a$, this can be approximated as $$\left(1-\frac{a^2}{2s_g^2}\right)v_{gs}$$ As an example, at 1000 feet above and 1 nautical mile horizontally from the station, this introduces an error of only about 1%. The error gets larger quickly as you get closer to the station.

If the calculation is based purely on DME then yes slant range makes the DME groundspeed inaccurate.
However, DME groundspeed is a calculation not a measurement, so the computer could include DME station elevation and aircraft altitude in the calculation and remove the slant range error. In practical terms, significant error would remain when very near the station due to limits in the instrument precision and unfavorable trigonometry. The actual amount of error and zone of significant error would depend on the specific equipment.

In a stand-alone DME unit the computer (to the extent the simple circuitry in there can even be called a computer) could not include the DME station elevation, because it has no way to know which DME station it actually is.

A DME receiver and a groundspeed computer are different devices, even if they happen to be packaged into one box. There are analog computers too. Anyway, how the information could be provided to the computer is not relevant. It could be a database or manual entry or something else. The user manual has all this info and more, including the known errors of the particular model number.

How the information could be provided is very relevant, because it is a lot of information. That information is available in a flight management computer—that's it's purpose—but it wouldn't be a stand-alone DME, or even a stand-alone NAV radio, any more if it was connected to all the other data sources needed.


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Why does the FAA allow a dual receiver check for IFR
FAR 91.171 allows a pilot to use independent VORs to do the VOR check as opposed to using a testing facility. The reg says as long as they are within 4 degrees of each other the pilot is good to go. But why do they allow this? What if they were both within 4 degrees of each other but both were also off about 45 degrees when actually tuning to a VOR. It seems like using the dual VOR check tests the precision relative to another unit but not the actual accuracy of the system.

Not a mathematician, but I'd guess there are some probability calculations involved.  Two independent units both being 45 degrees off, yet within 4 degrees of each other seems extremely unlikely.

Remember that what you are testing is not “the system” but only your receivers.  The VORs themselves are continuously monitored and kept in precise calibration.  You are testing two receivers that share no common components save for power supply.  A bit of trivia;  VORs do not work at all the way they are described in pilot training.  There’s no “fixed pulse” and series of rotating pulses that arrive at different times.  It’s actually two continuous radio waves that arrive out of phase, creating a third “net” frequency when compared. That discrete frequency equates to a radial.

You're correct that a dual check does not directly assess the accuracy of either of the individual systems to the true radial the aircraft is located on.
However, the probability that your example of both of them being in error by 45 degrees and also within 4 degrees of each other at the same time is improbable.  It is likely that the FAA has examined the risk associated with the different methods of verifying the VOR's accuracy and determined that this is an acceptable risk due to the low chance of such a scenario coming to fruition.
It's important to remember that each VOR is a separate system, and they do not share common receivers.


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Why is a pressure type sensor fire warning system not affected by bends or kinks in the line?
In a pressure based fire warning sensor, there is a line containing helium gas with a hydrogen core. Flight manuals say that a bend or kink in the gas line does not affect their operation. But if there is a bend and kink in the line that is severe enough, wouldn't that prevent expanding gas further down the line from making it through the twisted part and activating the responder assembly?

It would take what amounts to a fold or crimp that provides a 100% gas seal. You'd have to fold the loop over and squeeze it together with pliers, and even then you might not get a complete gas seal.
Just bends and even kinks (sharp bends but not folds) from handling, or bumping into the sensor loop while working on it, can't create that kind of gas tight blockage.


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How much does each engine on a Boeing 747 weigh?
The Boeing 747 weighs 404,600 lbs in total, but how much of that does each engine take up?

It really depends on the 747 variant. You should precise which variant you are talking about.

Boeing 747 can be operated with different types of engines (and in general a specific model has multiple models of engines compatible with it due to competition between different engine manufacturers). Here are some of the engines that are compatible with B747 and their weights:

GE CF6-50: 8,825–9,047 lb
Rolls Royce RB211-524G/H : 12,540–12,764 lb
Pratt & Whitney PW4000-94: 9420 lb

So as you see, from model to model the engine weight might differ and all these engines have been used on B747 at some point.

According to Wikipedia, the PW4000 variant used on the 747 (PW4000-94), only weighs 9420lbs. The heavier variant is used on the 777.

I was not aware of this, thanks for bringing it up,

@Eylul Sure thing! I am glad I could help.


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Do pilots eat the same food as passengers?
I imagine it might be hard to find a time when pilots can eat, but when they do, what do they eat?

Three questions, three duplicates. Can I suggest that you explore the capabilities of the search bar at the top of the page before posting again?

Yes, commercial pilots eat the same meals that the passengers do. They may have extra snacks and such that the passengers don't, but that's it.
It's actually quite easy to find time to eat, since the majority of the actual control manipulation is handled by the autopilot. If the autopilot isn't available for whatever reason, one person can fly while the other eats.
Fun fact: Most airlines have a policy that the pilots must eat different meals, so that if there was a bad batch of chicken that gave everyone who ate it food poisoning, then the pilot that had the fish can still fly and land the plane.

Every now and then it actually happens that a pilot gets sick from food, so the practise of eating different meals is a really good one.

You will find more and more pilots and flight crew packing their own meals. Especially at the regional level. Younger pilots and health-minded pilots are opting for healthier choices than airline or airport food. Not a bad idea considering your health is your livelihood in a manner of speaking.

One possible source for the different meals policy is this: https://en.wikipedia.org/wiki/Flight_into_Danger and its more recent parody Airplane!